Add unit test

tools/engine_tool/lib/src/commands/test_command.dart

@@ -72,7 +72,7 @@ et test //flutter/fml:fml_benchmarks  # Run a single test target in `//flutter/f
 
     final List<BuildTarget> testTargets = <BuildTarget>[];
     for (final BuildTarget target in selectedTargets) {
-      if (target.testOnly && target.type == BuildTargetType.executable) {
+      if (_isTestExecutable(target)) {
         testTargets.add(target);
       }
       if (target.executable == null) {
@@ -102,4 +102,9 @@ et test //flutter/fml:fml_benchmarks  # Run a single test target in `//flutter/f
     }
     return await workerPool.run(tasks) ? 0 : 1;
   }
+
+  /// Returns true if `target` is a testonly executable.
+  static bool _isTestExecutable(BuildTarget target) {
+    return target.testOnly && target.type == BuildTargetType.executable;
+  }
 }

tools/engine_tool/test/test_command_test.dart

@@ -67,4 +67,28 @@ void main() {
       testEnvironment.cleanup();
     }
   });
+
+  test('test command skips non-testonly executables', () async {
+    final TestEnvironment testEnvironment = TestEnvironment.withTestEngine(
+      cannedProcesses: cannedProcesses,
+    );
+    try {
+      final Environment env = testEnvironment.environment;
+      final ToolCommandRunner runner = ToolCommandRunner(
+        environment: env,
+        configs: configs,
+      );
+      final int result = await runner.run(<String>[
+        'test',
+        '//third_party/protobuf:protoc',
+      ]);
+      expect(result, equals(1));
+      expect(testEnvironment.processHistory.length, lessThan(3));
+      expect(testEnvironment.processHistory.where((ExecutedProcess process) {
+        return process.command[0].contains("protoc");
+      }), isEmpty);
+    } finally {
+      testEnvironment.cleanup();
+    }
+  });
 }